Nuclear fuel assemblies, for powering nuclear reactors, generally consist of large numbers of fuel rods contained in discrete fuel rod assemblies. These assemblies or cells generally consist of a bottom end fitting or nozzle, a plurality of fuel rods extending upwardly therefrom and spaced from each other in a square pitch configuration, orientating or support grids spaced along the length of the assembly, a plurality of control guide tubes interspersed throughout the rod assembly, and a top end fitting or cap. The assembly is installed and removed from the reactor as a unit.
When the fuel rods have expended a large amount of their available energy, they are considered to be "spent" and the fuel rod assembly is pulled from the reactor and temporarily stored in an adjacent pool until the assemblies are transported to a reprocessing center or to permanent or temporary storage. Even though the rods are considered "spent" they are still highly radioactive and constitute a very real hazard both to personnel and to property
In general, there are a number of alternatives available for disposition of the fuel assemblies, none of which is totally satisfactory. The fuel assemblies can be enclosed in a suitable basket and cask arrangement and shipped to a storage facility, or possibly, to a reprocessing plant. This is an expensive process, and since re-processing of fuel has been deferred by the United States government, limited as to possible destinations.
A second alternative is to store the spent fuel in a dry storage system. Dry storage entails either the use of a large number of metal casks or the building of massive concrete containers either above or below ground, which is a very expensive process, and, where the storage system is above ground, not very conducive to the peace of mind of the indigenous population. A third alternative, and the one to which the present inventions is directed, is the storage of the fuel units in the existing water pool originally designed for temporary storage. This type of storage is the simplest and cheapest, since the fuel rod assemblies can remain in the pool and be left there until the appropriate governmental agency collects them, often at the end of the life of the nuclear plant. However, such storage pools have a limited capacity, and, where they are adjacent to the reactor, necessitate the construction of a new pool when one becomes full.
Numerous attempts have been made to increase the capacity of a pool through a process known as fuel rod compaction or consolidation. This process, in brief, comprises removing the rods from the assembly and placing them in a storage canister where they are placed in racks with minimal spacing. It is possible, with this process, to place the rods from two or more fuel assemblies into a single canister, thereby achieving approximately a 2:1 reduction in required pool volume. However, successful consolidation has been an elusive goal for a number of reasons. Since the pools are approximately forty feet deep, and inasmuch as the rods must remain immersed at all times, all of the consolidation operations must be performed under the shield and cooling water. In addition, even though the rods are kept under water, the process could be quite hazardous to personnel performing the operation.
Prior art arrangements for achieving rod consolidation have included a system whereby the rods are pulled out row-by-row, as in, for example, a 14.times.14 matrix of rods, lifted and deposited in a tapered interim storage container, which tapers from a large area top opening to a bottom that has the area of a storage canister. After the intermediate container has the rods from approximately two fuel assemblies deposited therein, the intermediate container is placed over a storage canister, the bottom of the tapered container is lowered to cause the rods to slide into the storage canister. If the rods jam or stick, as they often do, they must be pushed from above the pool by long rods. This last operation is made more difficult in that the rods develop on their outside surfaces what is referred to in the trade as "crud". When the rods are pulled, this radioactive crud is scraped off and clouds the water making it difficult for the operators to see what they are doing and contaminating the pool. The method just described has proven to be quite slow and complicated, and could be hazardous to personnel. Various other prior art systems and method have been developed, none of which has proved to be wholly satisfactory.